Very nice pictures.
I've checked these in detail with the Osang maker M.1915 GR helmet in my collection. The markings are more or less identical with regard to their positions (except for the 1916 date). My helmet has 'GR' stamped to the left hand side of the inside of the front peak trim.
Of interest, mine is also leather lined to the inner face of the rear peak and also has the pressed/stamped reinforcing "spine". Mine would also appear to be an issued item (as opposed to private purchase).
I've posted a few photos of mine on Col Joe's. (see M.1915 Grey metal Wappen) on Joe's excellent site.
Regards
Martin.